Claire Benton said at the end of the walk last year Thomas said he wanted to do the big one next time.
"I asked him again this year and he has stuck to it. It is so nice to be able to do it as a family with my mother coming up from Masterton and a way we can all give back," Mrs Benton said.
Mrs Redvers said Thomas has always been keen on walking and running and it was great he was putting it towards such a good cause.
"He told me he was doing it and said to me 'so how about it Granny'. I always want to support the grandchildren and it will be lovely walking with the family."
CanTeen youth co-ordinator for the Hawke's Bay office, Kerrie Waby, said what Thomas wants to set out and do with his family was "amazing".
"His commitment is fantastic and for someone who is only 8 years of age, it is wonderful."
The trio have been training on the weekends and Mrs Benton said she and Thomas have been doing walks up Te Mata Peak, to Havelock North and Clive.
"We have spent some quality time together and the process of planning how we will achieve our goal is great and we are doing something different."
Mrs Redvers said she had been walking around Masterton clocking up some kilometres.
"It will be great completing the walk and I think it is also about companionship, talking to one another and seeing the sights together."
Mitre 10 Mega manager Graeme Ricketts said they hoped to crack the 1500 mark this year with entries.
"It is good having four different lengths as there is something for everyone regardless of ability and distance."
The different lengths include a 28km, 15km, 6.5km and 3.5km run or walk.
The Mega Walk has supported CanTeen for six years now and has raised over $100,000.
"CanTeen does not get government funding but the money they do raise stays in Hawke's Bay, which is great."
The routes are the same as last year but Mr Ricketts said there will be a lot more activity at the finish line.
"We are hoping to get some Magpies players along with the inflatable goal posts from the Mitre 10 Cup, which will be amongst the face painting, bubble zone and bouncy castle."
The day begins with the 28km run or walk at 7.30am from Mitre 10 Mega in Hastings and finishes at 12.30pm with prize giving at the Napier store.
People can register at both of the Mitre 10 Mega stores in Hastings and Napier or online at mitre10.co.nz.
To donate to Thomas' walk go on to give.everydayhero.com and search 'Thomas Benton'.